Thermal concerns in semiconductor integrated circuits or discrete power devices need a solution for efficient heat transfer from the vicinity of the transistor channel, where hot electrons are generated.
Efficient thermal management by means of a high-thermal conductivity path through the substrate of a GaN-on-Si HEMT requires several considerations. Providing such a path by growth of GaN device layers on SiC substrates has been the most widespread practice. Excellent power device performance has been obtained by multiple research groups. However, thermal considerations cannot be completely mitigated by this approach due to the large thermal boundary resistance between the SiC substrate and the GaN nucleation layers. As a result, device engineers have faced a dilemma: power performance has been either curtailed in order to improve reliability, or maximized at the expense of optimal reliability.
Nanocrystalline diamond (NCD) has been of interest for a number of applications owing to its attractive properties such as high thermal conductivity, optical transparency, hardness, and inertness, among others. See O. A. Williams, Diamond & Related Materials Vol. 20 (2011) 621-640; see also J. E. Butler and A. V. Sumant, Chem. Vap. Deposition 2008, 14, 145-160.
For applications requiring a thin NCD film (e.g., AFM tip coatings, heat spreaders), NCD films having a thickness of less than 1 μm are typically grown with rms roughness in the 20-100 nm range. See Butler, supra; see also M. J. Tadjer, T. J. Anderson, K. D. Hobart, Tatyana I. Feygelson, Joshua D. Caldwell, Charles R. Eddy, Jr., Fritz J. Kub, James E. Butler, Bradford Pate, and John Melngailis, “Reduced Self-Heating in AlGaN/GaN HEMTs Using Nanocrystalline Diamond Heat-Spreading Films, IEEE Electron Device Letters, Vol. 33, No. 1, pp. 23-25 (2012) (“Tadjer et al., 2012”), which shares several authors in common with the present inventors, and which is hereby incorporated by reference into the present disclosure in its entirety.
Several groups have demonstrated GaN devices on polycrystalline or single-crystal diamond substrates.
For example, one approach involves III-Nitride growth on a large-area CVD diamond substrate growth. See, e.g. K. D. Chabak, J. K. Gillespie, V. Miller, A. Crespo, J. Roussos, M. Trejo, D. E. Walker, Jr., G. D. Via, G. H. Jessen, J. Wasserbauer, F. Faili, D. I. Babic, D. Francis, and F. Ejeckam, “Full-wafer characterization of AlGaN/GaN HEMTs of free-standing CVD diamond substrates,” IEEE Electron Device Letters, vol. 31, no. 2, pp. 99-101, February 2010. However, in this approach, substrate development may be incompatible with a CMOS process, and issues such as wafer bow have to be overcome.
III-Nitride structures have also been grown on a small-area, high quality single crystal diamond substrate. See K. Hirama, M. Kasu, and Y. Taniyasu, “RF High-Power Operation of AlGaN/GaN HEMTs Epitaxially Grown on Diamond,” IEEE Electron Device Letters, vol. 33, no. 4, pp. 513, 2012. However, the small substrate size required in this approach is incompatible with cost-effective device solutions, and is also incompatible with CMOS devices.
In another approach, a thin CVD diamond layer has been grown mid-process during III-Nitride device fabrication. See Tadjer et al., 2012, supra. Although the heat spreading layer in such an artifact can be very efficient due to its proximity to the heat source, advanced process development techniques need to be developed to avoid damaging the surface of the III-Nitride material. In addition, the diamond thickness relative to the rest of the transistor layers may be prohibitive to device scaling.
Others have investigated the growth of a thin CVD diamond cap on a fabricated III-Nitride device. See M. Seelman-Eggebert, P. Meisen, F. Schaudel, P. Koidl, A. Vescan, and H. Leier, “Heat-spreading diamond films for GaN-based high-power transistor devices,” Diamond and Related Materials, Vol. 10, no. 3-7, pp. 744-749, 2001. However, gate performance can degrade in such devices due to exposure of high temperature during CVD diamond growth.
Finally, some have utilized deposition of heat spreading materials other than diamond in the vicinity of a III-Nitride device. One material that has been used is graphene. See Z. Yan, G. Liu, J. M. Khan, and A. A. Balandin, “Graphene quilts for thermal management of high-power GaN transistors,” Nature Communications 3, 827, 2012. However, graphene is electrically conductive as well as thermally conductive, and so must be placed away from the device in order to avoid the introduction of parasitic current leakage paths.
Recent improvements in substrate surface preparation have enabled the exploration of NCD growth in vertically etched cavities. See T. I. Feygelson, T. J. Anderson, M. P. Ray, K. D. Hobart, and B. R. Pate, “Detonation versus laser-synthesized nanodiamond powders for seeding” 22nd European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes and Nitrides, Garmisch-Partenkirchen, Germany, 2011; and K. D. Hobart et al., International Conference on Diamond and Carbon Materials, Granada, Spain, 2012.
Metal-filled TSVs have been employed in advanced interconnect applications such as off-chip interconnect in multi-chip modules. TSVs coated or filled with NCD have the potential of providing electrically insulating thermal paths across semiconductor surfaces, and thus are of interest in semiconductor device thermal management applications.